The present invention relates to microwave devices generally and more specifically to matched impedance waveguide termination devices used for absorbing high microwave power propagated down a waveguide transmission line.
In high power microwave applications, it is often necessary to terminate a transmission line with a substantially matched load capable of absorbing and dissipating the power transmitted into the load. Methods of terminating a waveguide transmission line have been developed involving solid materials as the power absorbing medium, however, in most cases the absorbing medium is water or a water mixture. Where such fluid is used the general class of termination devices is generically referred to as “water loads”.
In designing a water load it is usually desirable to produce a load with suitable high power handling capability, low power reflection (i.e. low VSWR characteristics), broadband frequency operation, and relative simplicity of manufacture. It is also desirable to achieve these objectives for applications involving high pulse and high average power, typically pulse power levels in the range of megawatts to tens of megawatts or higher over the bandwidth desired, and average power levels as high as kilowatts to hundreds of kilowatts over the bandwidth desired.
A water load that generally meets these objectives at given power levels and frequencies is disclosed in U.S. Pat. No. 4,516,088 to Ray M. Johnson. The Johnson patent discloses a water load wherein water is circulated in a rectangular termination waveguide through a tapered dielectric jacket whose point end is inclined toward the narrow wall of the termination waveguide such that it lies in a region of low electric field strength for the waveguide's fundamental TE01 mode. By inclining the dielectric taper carrying the power absorbing water, reflected power can be minimized.
However, as increasingly higher power amplifier and oscillator sources are developed at both lower and higher frequencies, increased heating rates of the water jacketing material and the water within are experienced to the point where existing water load designs become incapable of handling the increased heating rates without deleterious and even catastrophic heating effects. A need therefore exists for a water load design, which reduces heating rates within the load and which consequently increases the water load's power handling capability. A need particularly exists for a water load which provides an increased power absorbing capability and produces relatively low reflected power over the bandwidth of the absorbed microwave power. A need still further exists to provide these characteristics in a water load designed for relatively high frequency applications (X-band and higher), where, as later described in greater detail, the absorption capacity of the water flowing through the water load is diminished.
It is also desirable for certain applications to provide a water load which maintains its performance for all orientations of the guide and for modest changes in fluid temperature. This means that the fluid flow characteristics should be maintained and entrapment of air bubbles prevented for all load orientations; otherwise there will be a deleterious effect upon the amount of power reflected by the termination. Heretofore, water loads have been devised that substantially reduce entrapment of air bubbles and that exhibits low reflected power characteristics. However, such loads are not well adapted for many high power applications and particularly high frequency, broadband applications at relatively high power levels.
The present invention provides a water load for waveguide transmission lines with relatively low reflected power characteristics over the bandwidth of the input waveguide, and with improved power handling capabilities. The present invention also provides a n improved water load capable of handling high power in high frequency applications (generally X-band and higher). The accepted measure of reflective power in waveguide is given by the voltage standing wave ratio, commonly denoted “VSWR.” The present invention seeks to provide a water load for high power applications having a VSWR value less that 1.1 for the entire operating frequency range of the input waveguide.
The present invention is also intended to provide a high peak and average power absorbing termination which is mechanically small and easy to construct and assemble.